"good-bye" is an actual word. "failed-and" is two words improperly hyphenated.
But spellcheckers don't really seem to consider that one word, for some reason. I guess they think all dashes are the same -- hyphens, en dashes, em dashes, what's the big deal?
So I'm not really sure what Sigil is supposed to do about that.
You'll see the same problem in calibre, or for that matter the Firefox/Chrome edit box.
